Biaxially oriented polyester films are often used in printing, coating and metalizing applications such as magnetic tapes, thermal transfer ribbons, packaging, optical applications and in many other web converted products because of their favorable properties. These properties include thermal stability, dimensional stability, and chemical resistance. Biaxially oriented polyester films are particularly frequently used for carrier films in transfer processes such as in-mold stamping, hot stamping and similar procedures.
Recently, end-users have desired improved image characteristics from stamped or molded parts. In particular, the desire for high gloss and precision stamping of an image on the surface of the transferred parts has increased. Furthermore, for molded parts used in electronic display applications, such as cellular phones or PDAs (personal digital assistants), there is a desire for a smooth surface on the display in order to produce a superior image. To achieve such image and surface attributes, the surface of the carrier biaxially oriented polyester film is preferably smooth. However, the smoother the surface, the higher the friction of the film, which makes the films more difficult to handle. Especially in the in-mold stamping processes, it is customary to provide a very smooth converting layer, such as a release layer, and a hard coating layer on the surface of the carrier biaxially oriented polyester film. These smooth converting layers can cancel the slippery effect of the biaxially oriented polyester film, and can make the carrier difficult to handle, which causes winding issues, blocking issues and other commercially important yield related issues.
There are a plethora of examples of biaxially oriented polyester films for stamping or molding in the prior art. However, these described structures do not fully satisfy the requirements of both high smoothness and easy handling.
Published Japanese Patent applications JP A Sho-64-45699 and JP A Hei-3-253317 describe traditional and general concepts of polyester films for stamping or molding. Japanese Patent Application Number 2003289990 describes a mold release film constructed by laminating a mold release layer to at least one side of a biaxially oriented polyester film including a polyester based on 1,4-cyclohexanedicarboxylic acid and 1,4-cyclohexanedimethanol moities. Japanese Patent Application Number 2004223800 describes a polyester film for stamping having an anti-static layer on the surface of the mold contact layer in order to improve productivity and converter's yield. However, there is no quantitative description about any smoothness requirements in these applications.
Japanese Patent Application Number 200582785 describes a polyester film for transfer materials having an anti-static layer on the surface of the die contact layer in order to improve productivity and converter's yield, and having a smooth surface onto the parts in order to transfer a clear image on the parts. Particularly, the example 4 of the art discloses a polyester film including A/B/C 3-layer structure of which surface onto the parts has the smoothness of 3 nm SRa and 200 nm Rmax (SRt), of which surface onto the die has the roughness of 13 nm SRa. However, to achieve the highly glossy and ultra smooth transferred surface of the parts, smaller protrusions compared with the protrusion of the art are required on the surface of the carrier polyester film. Furthermore, the roughness of the surface onto the die must be controlled more in order to make less of an influence to the surface of the parts under the conditions of the molding process.
Films, for example two layer films, are disclosed in the following U.S. patents, the disclosures of each of which are totally incorporated by reference herein, U.S. Pat. Nos. 4,615,939; 4,732,814; 4,778,708; 4,798,759; 4,952,449; 5,069,962; 5,294,474; and 5,626,942. The appropriate components and process aspects of the each of the foregoing U.S. Patents may be selected for the present disclosure in embodiments thereof.